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Q factor measurement with a VNA

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icpinu

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Hi,

I am measuring the Q factor of a coil with a VNA using two methods.
1 Transmission
2. Reflection

Do I need to have an impedance match for the weekly coupled small coils connected to the VNA. According to what I understand, no impedance matching is needed.

Is this correct?

Many thanks
 

Re: Q factor measurement

No, you do not need an impedance match. But if it is a high Q component, it is hard to measure accurately. You need a good calibration and good test fixtures.
 
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    icpinu

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Re: Q factor measurement

Hi,

Many thanks for your quick reply!
Could you please elaborate a bit further why this is not needed? Or what would be the effect in the Q measurement if an accurate impedance match is done. I am measuring high Q coils.

Also, I read that the input and output loops need to be balanced to have the same return loss, if they are well balanced does this help to avoid or reduce measurement errors and possibly the need of having impedance matched coupling probes (small loops).

Thanks
 

Re: Q factor measurement

Well, if you simply put a series inductor into a fixture and measure S21,S11, and then try to curve fit, you will find that a slight difference in measuremen (like 0.1 dB) has a big effect on Q reading.

I prefer to take a capacitor of known Q, and make a resonator of it and the inductor under test. In resonance, the voltage across the inductor is multiplied, so it is much more obvious to make an accurate reading.
 

    V

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Re: Q factor measurement

Sorry for not specifyin it before but that is exactly what I am planning oN doing. In that case how does the known Q of the capacitor affect the overal Q of the resonating coil?

Thanks

---------- Post added at 20:36 ---------- Previous post was at 20:28 ----------

Sorry for not specifyin it before but that is exactly what I am planning oN doing. In that case how does the known Q of the capacitor affect the overal Q of the resonating coil?

Thanks

---------- Post added at 20:39 ---------- Previous post was at 20:36 ----------

Sorry for not specifyin it before but that is exactly what I am planning oN doing. In that case how does the known Q of the capacitor affect the overal Q of the resonating coil?

Thanks
 

Re: Q factor measurement

The Q of any half decent capacitor will be greater then 1000 times that of the coil. The traditionel way of measuring Q is to use a low value specially made resistor (.04 ohms and very NONinductive), inject a small current into this resistor and use it in series with the inductor with a air spaced tuning capacitor, measuring the voltage across the capacitor. Good for 30MHz, at higher frequencies a special reference inductor is used instead .
Frank
 
Many thanks chuckey.
 

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